The object of the present invention is a process for the preparation of sennosides A and B, wherein rheinanthrone-8-glucoside is oxidized with oxygen or an oxygen source to form sennosides A and B.
The sennosides are agents with laxative properties, which agents are present for example in the senna plants Cassia angustifolia and Cassia acutifolia, from the parts whereof, such as from leaves and pods, they may be isolated by extraction in the form of a raw mixture. The most important of the sennosides are the sennosides A and B of the formula ##STR1##
The sennosides A and B are stereoisomers and differ from each other as to the configuration at positions 10 and 10'. Their reduction products are for example rheinanthrone-8-glucoside of the formula ##STR2## and rhein-8-glucoside (A. Stolle et al., Fortschritte der Chemie organischer Naturstoffe, band 599, 1950, p. 248-269).
The sennoside raw mixture contains, in addition to the desired sennosides, also by-products, for example sennidins (corresponding aglycones), as well as sennoside degradation products, which by-products may give rise to undesired side-effects. The total amount of sennoside type compounds in the raw mixture is approximately 68%, the remaining part being other agents of plant origin.
From the patent and other literature a number of methods and method variants for the recovery of sennosides from senna plants are known. Thus, for example, various extraction methods and combined extraction-crystallization methods for the recovery of sennosides are known, see for example FI-patent 75992 and the literature reference listed therein.
It is also known (i.a. WO 93/00350) to extract senna pods and leaves with a methanol-water-mixture at room temperature and evaporate the methanol to prepare a sennoside raw mixture. In addition, the obtained concentrate can be washed, if desired, for example with an alcohol, such as 2-butanol. Thereafter the sennoside raw mixture obtained can be reduced to form rheinanthrone-8-glucosides. Various reduction reagents, such as stannous(II)chloride, sulphur dioxide, alkalimetal borohydride, and especially alkalimetal dithionite have been suggested for use as reduction agents.
In order to recover the sennosides it is known to oxidize rheinanthrone-8-glucoside back to form the corresponding sennoside compounds. As suitable reagents, i.a. hydrogen peroxide and manganese dioxide have been suggested, but also oxygen, such as atmospheric oxygen. It has also been suggested to facilitate the oxidation by adding a catalyst, palladium black or iron(III) salts having been suggested for this purpose.